Not Applicable
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of a contract awarded by an agency of the U.S. Government.
Not Applicable
1. Field of the Invention
The invention pertains to a method for the fabrication of microelectronic devices on substrates. More particularly this invention pertains to a method for the fabrication of layers of material in a specified, desired pattern upon a substrate, which patterned layers comprise the microelectronic devices or portions thereof. The method may be used for the fabrication of microwave acoustic resonators as well as other microelectronic devices.
For the purposes of this specification and the claims the terms xe2x80x9cdesired materialxe2x80x9d and xe2x80x9cdesign patternxe2x80x9d mean the desired, design pattern in which a desired material is to be fabricated upon a substrate. The term xe2x80x9cinverse patternxe2x80x9d means the negative, or complement of the design pattern. The meaning of the term xe2x80x9clayer of materialxe2x80x9d, in the appropriate context, also includes a layer of material from which some portions have been removed, e.g. where the portion of the layer included within the inverse pattern has been removed leaving the portion of the layer constituting the design pattern. The term xe2x80x9cphoto-resist materialxe2x80x9d also includes photo-imageable material. Although some of the following examples describe fabrication techniques that use positive photo-resist materials, it should be understood negative photo-resist materials could instead be used for these purpose and that the masks used for controlling the exposure of the photo-resist to ultraviolet light would then be inverted accordingly.
2. Description of the Prior Art
Methods for the fabrication of layers of desired material in design patterns upon a substrate are well known in the art. For example, a uniform layer of a desired material such as aluminum may be deposited upon a substrate by evaporation or other means and then covered by a layer of positive photo-resist material. The layer of positive photo-resist material is then exposed in the inverse pattern to ultraviolet light using a photographic mask. The exposed portions of the photo-resist material are then removed by application of appropriate chemicals in a developing process. Instead of ultraviolet light, other means such as electron beams could instead be used to render the portion of the photo-resist material so exposed susceptible to removal in the developing process. Chemical etchants, to which the remaining portions of the layer of photo-resist material are resistant, are then used to remove those portions of the layer of aluminum that are not protected by the remaining pattern of photo-resist material. Finally other chemical processes are used to remove the remaining photo-resist material, leaving upon the substrate a layer of aluminum, i.e. the desired material, having the desired, design pattern.
However, the fabrication process described above cannot be used in applications where the removal of the inverse pattern of the layer of aluminum, or other desired material, by etching or by other means would also remove other layers of the aluminum or other desired material previously deposited upon the substrate in those circumstances where one does not wish also to remove such other layers.
In such circumstances a second method in the prior art for fabricating layers of a desired material in a desired pattern on a substrate, commonly known as the xe2x80x9clift-offxe2x80x9d method, can be used. The lift-off method reverses the sequence in which the layers are applied to, and removed from, the substrate. As illustrated in FIG. 1a, in the lift-off method, a layer of positive photo-resist material 101 is first deposited upon substrate 100 and an upper layer 102 of the photo-resist material is hardened so as to make the unexposed portions of layer 102 more resistant to removal in the developing process as compared to the unexposed portions of layer 101. Such hardening can be achieved by chemical treatment of the upper layer 102 of the material or by depositing layer 102 in a separate step using a more resistant photo-resist material. Next the layers of photo-resist material are exposed to ultraviolet in the design pattern and the exposed portion 106, depicted as the shaded area in FIG. 1b, is removed by chemicals in a developing process. Because layer 102 is more resistant to removal, the chemical developing process that removed portion 106 also slightly undercuts the edge of the unexposed portion of layer 102 leaving the overhanging lip 103 depicted in FIG. 1c, which lip extends slightly beyond the edge 107 of underlying layer 101.
As indicated in FIG. 1d, a layer 104 and 105 of the desired material is next deposited over the substrate by evaporation or other means that transports the desired material in a directional manner as indicated by arrows 108 such that that none of the desired material is deposited on edge 107 of layer 101 of photo-resist material that is shadowed by lip 103. A chemical etchant that does not attack the desired material, but that does attack the photo-resist material 101 is then applied to complete the removal of layer 101 causing layer 105 to be lifted-off from the substrate leaving the desired material in layer 104 in the design pattern on the substrate as depicted in FIG. 1e. 
The essential element of the xe2x80x9clift-offxe2x80x9d process is the creation of an overhanging lip that shadows the underlying edge 107 of the unexposed photo-resist material from the deposition of the desired material so that a chemical that does not attack the desired material can then be used to remove the photo-resist material, thereby undercutting and lifting off layer 105 of the desired material, leaving layer 104 of the desired material in the design pattern on the substrate.
Unfortunately, for some desired materials, the high temperatures and the extended length of application times that are used in the deposition process polymerizes the photo-resist material such that the photo-resist material can no longer be removed by normal chemical processes so as to lift-off the overlying layers of material. The prior art lift-off method also may not be used if the deposition flux for the desired material is not well columnated. In such a circumstance, the narrow lip on the layer of photo-resist material may be insufficient to shadow the edge of the underlying photo-resist material and the underlying edge would be coated with the desired material. This coating would then prevent the removal by etching of the underlying layer of photo-resist material.
Instead of the photo-resist material used in the lift-off process described above, the method of the present invention uses a different material that is not polymerized or otherwise degraded by the high temperatures and extended periods used for the deposition of the desired materials. The different material can then still be removed by a chemical etchant so as to undercut and lift-off the overlying inverse pattern of material. The present invention thus provides a means for fabricating a desired material upon a substrate in a desired design pattern in circumstances where the high temperatures and/or extended deposition periods used to deposit the desired material upon the substrate would either substantially degrade or render unusable the prior methods of using photo-resist materials to fabricate the desired design patterns upon the substrate.
The present invention also provides a simple means for increasing the thickness of the layer of material that is removed by the chemical etchant so that the process may be used in circumstances where the combined thicknesses of multiple layers of desired materials deposited upon the substrate might otherwise equal or exceed the height of the overhanging lip used in the prior art lift-off process and, as a consequence, would prevent the underlying layer of material from being removed by the chemical etchant.
One embodiment of the present invention also provides a lip having a greater overhang that can be used to shadow a portion of the edge of the underlying material from the deposition of the desired material even when the deposition process is not fully directional. For instance, if the desired material is deposited using a process having other than very low partial pressures, molecular collisions will cause some material being deposited to follow different paths resulting in incomplete shadowing of the edge 107 from the desired material being deposited. By increasing the extent of the overhang, this embodiment of the invention can be used with such deposition processes.